JP2522349B2 - Film thickness measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention irradiates a sample with an electron beam and detects characteristic X-rays emitted from the elements constituting the sample to determine the film thickness of the sample. Regarding how to measure.

(Prior Art) An optical method can be applied to measure the thickness of the transparent film,
Not applicable for opaque samples. The method of irradiating a sample with an electron beam and measuring the intensity of the characteristic X-rays emitted from the sample can be applied to both metallic and non-metallic materials regardless of whether the sample is transparent or opaque. There is a feature that the material range of the sample that can be made is wide.

The principle of the method using the electron beam described above is as follows. When the sample is irradiated with an electron beam and the intensity of the characteristic X-ray of the sample component element emitted from the sample is measured, when the sample is thin, the characteristic X-ray intensity increases with the thickness of the sample and eventually becomes saturated. Therefore, when the sample is thin, the thickness of the thin film of the sample can be obtained from the relationship between the thickness and the intensity of the characteristic X-ray. In this case, it is necessary to calibrate the relationship between the thickness of the sample film and the characteristic X-ray intensity, and therefore at least one kind, and in some cases, several kinds of standard samples of known thickness are necessary.

(Problems to be Solved by the Invention) As described above, a standard sample is required in the conventional film thickness measurement method of irradiating an electron beam and measuring the characteristic X-rays emitted from the sample. Can be difficult to obtain.

Therefore, the present invention is intended to enable the above-described film thickness measurement method to be performed without a standard sample.

(Means for Solving the Problem) When the sample is irradiated with electron beams accelerated by two or more different accelerating voltages, the intensity ratio of one characteristic X-ray of the sample constituent element emitted from the sample and the sample The relationship with the thickness is obtained in advance, and the measured intensity ratio of the characteristic X-rays when the sample is irradiated with an electron beam accelerated by the same voltage as described above is used to calculate the relationship between the intensity ratio and the thickness of the sample. The film thickness of was determined.

(Action) When a given sample is irradiated with an electron beam accelerated by a constant accelerating voltage, the intensity of the characteristic X-rays of the sample constituent elements emitted from the layer of an arbitrary depth from the surface of the sample is the depth from the sample surface. It can be theoretically calculated as a function of d. The shape of this function is as shown in FIG. 2, and shows the maximum value at the depth of the sample. FIG. 2 shows the case of two electron acceleration voltages. The intensity X of the characteristic X-ray obtained when the sample is irradiated with the electron beam is proportional to the value obtained by integrating the function in the figure from depth 0 to t, where t is the thickness of the sample. This is shown in FIG. 3A. The depth at which the characteristic X-ray intensity emitted from the sample is maximum, dm or dm 'in Fig. 2 depends on the accelerating voltage of the irradiating electrons, and is shallow when the accelerating voltage is low as shown in the same figure. The one coming closer. There is a lower limit energy for the electrons to excite the atoms to emit the characteristic X-rays, but even if the energy is too large, the probability that the electrons excite the atoms in the substance decreases, so there is an optimum energy. On the other hand, the electrons that have entered the substance lose their energy while moving in the substance, and statistically, this is the optimum energy at a certain depth from the sample surface. Moreover, the number of arriving electrons decreases as the depth from the sample surface increases. Therefore, the characteristic X-ray emission intensity becomes maximum in a layer having a certain depth in relation to the acceleration voltage of the irradiation electrons, and the depth becomes shallower as the acceleration voltage becomes lower. The present invention utilizes this relationship.

FIG. 3A illustrates the relationship between the thickness of the sample and the characteristic X-ray intensity X in the case of two accelerating voltages, and FIG. 3B shows the ratio of the X-ray intensity in these two cases. The curve of this ratio can be calculated in advance because the function of FIG. 2 can be theoretically calculated. Then, for the sample to be measured, the intensity ratio when the accelerating voltage is changed is obtained from the measured value of the characteristic X-ray intensity when the sample is irradiated with electrons accelerated by the same voltage as used in the above calculation, The thickness can be determined from the curve in FIG. Since the ratio curve has a point of maximum value in the middle as shown in FIG. 3B, two thicknesses are obtained for one ratio value in the thickness range from 0 to t. I can't decide. Even in such a case, the thickness can be determined by using the actual measurement value of the intensity of the characteristic X-ray when the electron beam accelerated by the third voltage is applied. In FIG. 4, A is a curve of the ratio when the accelerating voltage is V1 and V2, B is a curve of the ratio when the accelerating voltage is V1 and V3, and shows a characteristic X when the accelerating voltage is V1 and V2.
When the strength ratio of the wire is K1, according to curve A, the thickness is
Two types, t1 and t2, are obtained. If the measured value of the characteristic intensity ratio when the accelerating voltage is V1 and V3 is K2, two thicknesses, t1 ′ and t2 ′, are obtained from curve B. The thickness t1 is determined from the agreement between t1 and t1 '.

(Embodiment) FIG. 1 is a flowchart of the operation when the method of the present invention is carried out using a computer. Enter the elemental composition of the film to be measured and the substrate supporting the film (a) and set the electron acceleration voltages V1, V2, and V3 to be used (b). For each acceleration voltage, sample thickness 1,2, ... i ... n (unit
The radiation X-ray intensities I1i, I2i, I3i are calculated for each (nm) (C).
(D) (e). Next, the intensity ratios (I2i / I1i) and (I3i / I1i) of the characteristic X-rays at each accelerating voltage and the sample thicknesses 1, 2, ...
... Create relation tables A and B with n (f) (g). This relationship table corresponds to curves A and B in FIG. When this table is created, the characteristic X-ray intensities X1, X2, and X-ray intensity of the sample are measured with EPMA using electron acceleration voltages V1, V2, and V3.
Measure X3 and enter (h) the result to have the thickness determined. That is, (X2 / X1) and (X3 / X1) are calculated (re),
The corresponding thicknesses t1, t2 and t1 ', t2' are searched out from the tables A, B (n) (ru), and the values closest to each other from the groups of t1, t2 and t1't2 ', for example, t1 and t1. ′ Is calculated and the average (t1 + t1 ′) / 2 is output as the film thickness of the sample.
To do.

The calculation of the characteristic X-ray intensity is performed as follows. The electrons incident on the sample travel in an irregular orbit while colliding with the atoms constituting the sample, gradually lose energy, and are finally absorbed in the sample or escape from the sample again. Such a process is probabilistic, and the frequency with which the atoms constituting the sample are excited and characteristic X-rays are emitted while the electron travels in the sample is also probabilistic. It depends on the initial accelerating voltage and the composition of the sample. Therefore, the probability of characteristic X-ray emission is calculated by simulating the movement of an electron when one electron is incident on the sample using those probabilities. The number of electrons is 1 to 2
The thickness and characteristics of the sample X
Find the relationship with the line strength. In this case, when the sample is thin, electrons penetrating the sample enter the substrate layer, and some of these electrons are reflected in the substrate layer and enter the sample layer again, and the characteristic X
Some of them emit a line, and the number and energy of electrons reflected from the substrate layer depend on the composition of the substrate layer. Therefore, the composition data of the substrate layer is also necessary for the calculation.

The X-ray intensity obtained by the above calculation is a relative value,
Since the measured X-ray intensity differs depending on the value of the electron beam current irradiating the sample, it is impossible to directly compare the calculated value with the actually measured X-ray intensity, but the ratio of the X-ray intensities at the two acceleration voltages is a relative value. Since the ratio is the same as the measured value ratio,
The ratio value can be directly compared with the calculated value and the measured value, and the film thickness can be obtained without the operation of determining the conversion ratio between the calculated value and the measured value using a standard sample.

(Effect of the invention) According to the present invention, the thickness of a thin film can be measured without using a standard sample by using an X-ray spectroscopic analyzer such as EPMA. Therefore, a standard sample cannot be obtained for technical or economic reasons. It is possible to measure the film thickness of such a sample.

[Brief description of drawings]

FIG. 1 is a flow chart of the operation of a computer for carrying out the method of the present invention, FIG. 2 is a relationship graph between the depth from the sample surface and the characteristic X-ray intensity emitted from the layer at that depth, and FIG. A graph of the relationship between the thickness and the characteristic X-ray intensity emitted from the sample, FIG. 3B shows the characteristic X when the accelerating voltage is different.
Graph of the relationship between line intensity ratio and sample thickness
It is a graph explaining the procedure which calculates | requires sample thickness from the graph of a figure.

Claims (1)

(57) [Claims] 1. A relationship between the sample thickness and the intensity ratio of one characteristic X-ray of a sample constituent element emitted from the sample when the sample is irradiated with electron beams accelerated by two or more different acceleration voltages. It is calculated in advance, the sample is irradiated with electrons accelerated at the same respective voltages as above, the characteristic X-ray intensities at the respective acceleration voltages are measured, and the ratio of these measured X-ray intensities is calculated in advance by the above-mentioned calculation. A film thickness measuring method, characterized in that the thickness of the sample is determined by the relationship between the obtained thickness and the strength ratio.